Sound And Light Training
SALT Manual

11. Understanding Sound

A Decibel (dB) describes a ration between two quantities
expressed as a logarithm. Logarithms are used because our ears
hear differences in loudness as a Log function.

In simple terms, 3 dB represents twice as much power, and 10
dB represents 10 times as much power

2 * power = 3 dB

4 * power = 6 dB

8 * power = 9 dB

10 * power = 10 dB

To make sound twice as LOUD requires 4 times as much power
which is 6 dB

If you have a 100 Watt stereo, and you want one that is
twice as loud, you need a 400 Watt stereo

Loudness of a sound system is measured in dB of Sound Pressure
Level (dB SPL)

Loudness decreases by 6 dB (half as loud) every time the
distance from the sound source is doubled

If you start 3 feet from the speaker and move back to 6
feet, it will sound half as loud (-6 dB)

If you move from 6 feet to 12 feet, it will reduce in volume
by half again

By the time you are 48 feet from the speaker, the sound will
be 1/16th as loud (-24 dB) as it was at 3 feet

Frequency Range of the Human Voice

Voice range covers 300 Hz to 3500 Hz

Most energy concentrated below 1000 Hz

Vowels have most of their energy below 1000 Hz

Vowels contain the "power and impact of the
voice"

Consonants have most of their energy above 1000
Hz

Consonants are responsible for intelligibility

Harmonics in voice can go above 3500 Hz

Poor high frequency response reduces
intelligibility

Recognizing the Frequency of Sounds

It is important for a sound technician to learn to recognize the
frequency of sounds so you are able to quickly and correctly
adjust equalization when there is a problem (i.e. feedback,
hollowness, nasal sound, boomy, etc.)

When you have time alone with a sound system, put on some good
quality music and play with the graphic equalizer to learn what
frequency range each control affects. Memorize the tonal quality
each control affects.

40-60 Hz

Boomy - a sound over abundant in low
lows. These waves move a lot of air, hence Boomy.

Sensitivity of most people's ears is relatively smooth
between 500 Hz and 5000 Hz

Ears are most sensitive to sounds between 3000 Hz and 4000
Hz

Below 500 Hz and above 5000 Hz, hearing sensitivity drops
off

At louder listening levels (rock concerts), the frequency
response of the ears becomes more equal over a wider
range

This is why you really can't notice the bass or cymbals in
quiet music, but they are quite evident in loud music

Many stereos have a "loudness" switch to compensate for this
effect at low volumes

A Sound Pressure Level (SPL) of 120 to 130 dB SPL is the
threshold of PAIN for most people

Children and women are more sensitive to loud sounds than
men

Dynamic Range

Singers and instruments are capable of performing as quiet
as 50 dB SPL and as loud as 110 dB SPL

This represents a 60 dB dynamic range from the quietest to
loudest

The quietest parts of a performance must still be kept louder
than the room noise (called the noise floor)

The noise floor is typically 50 to 60 dB SPL

The quietest parts must be amplified enough that they can
still be heard above the noise floor in the back of the
room

Assuming 24 dB of loss from the front to the back of a 50 foot
long room, the quietest parts would need to be amplified 24 dB to
be heard in the back of the room

Note: It may not be possible to provide 24 dB of gain before
feedback (see Feedback Control)

The speakers must reproduce the quietest parts at 74 dB SPL in
order to be heard at the back of a 50 foot room at 50 dB
SPL

The loudest parts of the performance must not be so loud as to
be obnoxious or painful

The loudest should not exceed 110 dB SPL

The loudest cannot exceed the capabilities of the amplifier
and speaker system (also about 110 dB SPL)

However, since the quietest parts must be amplified 24 dB, the
loudest parts (110 dB SPL) will also be amplified the same amount,
obviously making them much TOO LOUD

The useful dynamic range of the speaker system is limited
by:

The quietest part of the program must be amplified to 74
dB SPL to be heard in the back of the room

The loudest part of the program exceed 110 dB SPL in the front
of the room

This leaves a useful dynamic range of 110 - 74 or 36 dB, much
less than the 60 dB dynamic range of a typical music
group

Solutions to the Dynamic Range problem:

Make the room quieter (requires expensive sound
insulation)

Get a louder sound system (could annoy the
audience)

Use a compressor / limiter circuit (expensive, especially in a
system with many microphones and instruments in one
system)

Have the sound engineer turn quiet parts up and turn loud
parts down

***** Have the performers reduce the demand for dynamic range
by getting closer to the mike on quiet parts and backing off from
the mike on loud parts, similarly, have instrument players control
their own volume according to the dynamic needs of the
program

Feedback Control

Feedback occurs when the sound from the microphone is
amplified too much.

Feedback is caused by a repeating circular process of a
microphone picking up a sound from a speaker, the sound system
amplifying it (too much), the speaker reproducing the sound again,
and the microphone picking it up again.

Feedback usually occurs at one frequency at a time. The
frequency of the feedback is affected by:

Direction the microphone is facing

Distance between the microphone and speaker

Frequency response characteristics of the room

Equalization of the microphone channel

Equalization of the monitor speakers

Equalization of the main speakers

Sound systems should be operated no louder than 6 dB before
the beginning of feedback (that is half as loud as when feedback
starts)

Operating closer to the feedback point causes a "hollow" or
"ringing" sound

Adding more microphones increases feedback problems. Every
time you double the number of microphones, the maximum gain before
feedback is reduced by 3 dB. Apparent loudness of each mike is cut
in half when the number of mikes is increased 4
times.

To go from 1 to 4 mikes halves the maximum volume of each mike
before feedback.

To go from 1 to 16 mikes quarters the maximum volume of each
mike before feedback.

Feedback is controlled by:

Using directional microphones and carefully aiming them
away from monitor and main speakers to reduce
feedback

Performers must be careful not to re-aim mikes towards
speakers during performance

Performers must be careful when hand holding mikes not to
point them towards the monitor speakers

Decreasing the distance between the sound source (performer)
and the microphone (so the microphone does not need to be as
loud)

Increasing the distance between the speakers and the
microphones

Using equalizers to reduce the system's gain at the
frequencies where the feedback occurs

Installing acoustic dampening material in the room to reduce
sound reflections back to the microphones (expensive
solution)

Factors Influencing Clarity and Intelligibility

High monitor levels on stage get into the microphones and
muffle the sound because the monitor sound is out of phase with
the original sound (don't set monitor louder than
necessary)

High monitor levels can also cause sound to be hollow or
ringing

Instruments playing music louder than necessary on stage
causes too much music to be picked up by vocal mikes, making the
music muddy (keep music as quite as possible on
stage)

Excessive use of equalization to prevent feedback effects the
clarity of the overall sound (again, don't set monitor louder than
necessary)

Acoustical characteristics of the room (reverberation) affects
the intelligibility of the sound. Large flat hard surfaces reflect
sound which is out of phase with the original sound. Acoustical
treatment of walls and ceilings is desirable.

When performers are too far from the mike, the mike gain must
be increased which causes pickup of more background noise and
muddies the sound (the optimum distance from the microphones is 6"
except for solos where "quiet" singers should be 2" from
mike)

Microphones aimed at monitor speakers, instrument speakers, or
drums (tilt mikes up slightly so they point at a "quiet" ceiling
rather than behind you at a guitar amp or drummer)

Breath noises and "popping" on solos (tilt mike up and
sing/speak over the top of it, stay 6" from mike)